Method of removing nitrogen oxides from a gas flow

ABSTRACT

The invention relates to a process for removing nitrogen oxides from a gas stream containing same, which comprises passing the gas stream 
     (A) through a stage for absorbing the nitrogen oxides other than N 2  O in an absorbent or reacting the nitrogen oxides other than N 2  O with an absorbent at a pressure of from 1,5 to 20 bar, and 
     (B) through a stage for reducing the amount of N 2  O, preferably employing the pressure level of step A, and to apparatus therefor and the use thereof.

The present invention relates to a process for removing nitrogen oxidessuch as NO, NO₂ and N₂ O from a gas stream containing same. Nitrogenoxides are formed as by-products in many processes in which HNO₃ is usedas oxidizing agent in liquid phase. Especially the conversion ofalcohols, aldehydes and ketones, for example the conversion ofcyclohexanol and cyclohexanone into adipic acid, of acetaldehyde intoglyoxal or of glyoxal into glyoxylic acid, and also the production ofnicotinic acid and hydroxylamines liberate for example appreciableamounts of N₂ O as well as other nitrogen oxides.

In Science 251 (1991), 932, Tmiemens and Trogler show that N₂ O has acertain destructive potential for the Earth's atmosphere. N₂ O serves asthe major stratospheric source of NO, which in turn has an essentialinfluence on the depletion of ozone in the stratosphere. In addition, N₂O is considered a greenhouse gas whose global warming potential is saidto be about 290 times greater than that of CO₂.

Recent years have witnessed the publication of a multiplicity of patentand non-patent documents concerned with reducing the N₂ O emissions dueto anthropogenic sources.

A multiplicity of patents describe catalysts for reducing or decomposingN₂ O, for example DE 43 01 470, DE 42 24 881, DE 41 28 629, WO93/15824,EP 625369, WO94/27709, U.S. Pat. No. 5,171,553.

U.S. Pat. No. 5,200,162 discloses that the exothermic reaction of thedecomposition of N₂ O into nitrogen and oxygen can lead to amultiplicity of process problems associated with high processtemperatures. It describes a process for decomposing N₂ O in a gasstream by contacting an N₂ O-containing gas stream under N₂ Odecomposition conditions with a catalyst for decomposing N₂ O intonitrogen and oxygen by first cooling part of the exit gas whose N₂ Ocontent is reduced and then recycling it into the N₂ O decompositionzone. In the case of N₂ O-containing waste gas streams containingadditional NO_(x) it is stated to be frequently very desirable topretreat the gas stream to remove NO_(x) upstream of the N₂ Odecomposition zone by selective reduction of NO_(x) with ammonia in thepresence of oxygen.

In Abatement of N₂ O emissions produced in the adipic acid industry,Environmental Progress 13 (1994), No. 2, May, 134-137, Reimer, Slaten,Seapan, Lower and Tomlinson describe a boiler gas reburn system coupledwith selective non-catalytic reduction (SNCR) for destroying N₂ O. Aflow diagram of the catalytic decomposition of N₂ O shows an N₂ Odecomposition catalyst stage coupled with an NO_(x) abatement SCRcatalyst stage.

Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, volume A17,1991, pages 293-339, describes the production of HNO₃ by burning ammoniaand absorbing the combustion products in water. Nonselective catalyticreduction (NSCR) and selective catalytic reduction (SCR) processes canbe used for treating the waste gases from the HNO₃ production process.

It is an object of the present invention to provide a process forremoving nitrogen oxides from a gas stream containing same.

It is a further object of the present invention to provide a process forremoving nitrogen oxides from a gas stream containing major quantitiesof N₂ O as well as other nitrogen oxides.

It is a further object of the present invention to provide a process forremoving nitrogen oxides from a gas stream containing same to producenitric acid (HNO₃).

It is a further object of the present invention to provide a process forremoving nitrogen oxides from a gas stream containing same under simpleconditions.

It is a further object of the present invention to provide an apparatusfor the aforementioned processes.

We have found that these objects are achieved by the processes andapparatus claimed in the claims.

The term "nitrogen oxides" as used in the description and the claimsdesignates the oxides of nitrogen, especially dinitrogen oxide (N₂ O),nitrogen monoxide (NO), dinitrogen trioxide (N₂ O₃), nitrogen dioxide(NO₂), dinitrogen tetroxide (N₂ O₄), dinitrogen pentoxide N₂ O₅),nitrogen peroxide (NO₃).

The present invention provides in particular a process for removingnitrogen oxides from gas streams as obtained for example as waste gasstreams in processes for producing adipic acid, nitric acid,hydroxylamine derivatives, caprolactam, glyoxal, methylglyoxal,glyoxylic acid or in processes for burning nitrogenous materials.

The aforementioned processes as well as other processes for oxidizingorganic compounds with nitric acid give rise to reaction productscontaining nitrogen oxides. For instance, the production of adipic acidby oxidation of a cyclohexanone/cyclohexanol mixture gives rise to awaste gas having, for example, the following composition:

    ______________________________________                                        NO.sub.2            20% by volume                                             N.sub.2 O           23% by volume                                             O.sub.2             10% by volume                                             CO + CO.sub.2        2% by volume                                             N.sub.2 + Ar        45% by volume                                             ______________________________________                                    

According to the present invention, the removal of nitrogen oxides aspresent for example in the aforementioned composition is effected bypassing the gas stream

A) through a stage for absorbing the nitrogen oxides other than N₂ O inan absorbent or reacting the nitrogen oxides other than N₂ O with anabsorbent, and

B) through a stage for reducing N₂ O.

Preferably, the gas stream passes first through stage A and then throughstage B.

Stage A

The absorption of the nitrogen oxides other than N₂ O in an absorbentand the reaction of the nitrogen oxides other than N₂ O with anabsorbent, as the case may be, can be carried out with any desiredsuitable absorbents. The preferred absorbent is water or an aqueoussolution, e.g. of nitric acid, in which case the absorption ispreferably carried out in the presence of free oxygen and the nitrogenoxides other than N₂ O are preferably converted into HNO₃.

In particular, for example, nitrogen monoxide is oxidized to nitrogendioxide and nitrogen dioxide is absorbed in water to form HNO₃. Such aprocess is described in Ullmann's Encyclopedia of Industrial Chemistry,5th edition, volume A17, 1991, pages 293-339.

The process for the conversion into nitric acid can be characterized bytwo exothermic reaction steps:

oxidation of nitrogen monoxide with atmospheric oxygen to nitrogendioxide according to:

    2NO+O.sub.2 →NO.sub.2                               (I)

absorption of nitrogen dioxide in water and reaction according to:

    3NO.sub.2 +H.sub.2 O→2HNO.sub.3 +NO                 (II)

The reactions are promoted by high pressures and low temperatures.Pressures of 1,5 to 20 bar, preferably from 3 to 12 bar, particularlypreferably from 5 to 10 bar are employed.

The gas inlet temperature on entry into stage A is preferably from 10 to100° C., particularly preferably 20-60° C., in particular from 30 to 40°C.

The gas streams from the oxidation of alcohols, aldehydes and ketonesoften contain NO₂ in a concentration of more than 1% by volume, so thatthe NO₂ can be considered not an impurity but a material of value andtherefore can be converted into nitric acid by reaction with water.

The reaction can take place in absorption columns and is described forexample in Ullmann's, loc. cit.

The heat produced in the exothermic reaction can be utilized forgenerating process steam and/or for heating the gas streams containingnitrogen oxides, for example in a gas/gas heat exchanger.

Stage B

Stage B is a stage for reducing the amount of N₂ O.

The reduction of the amount of N₂ O can be effected by thermal and/orcatalytic decomposition. The process can be carried out adiabatically orisothermally, preferably employing the pressure level of process step A.

The removal of N₂ O can be carried out in various ways, for example byheterogeneous catalysis. In the adiabatic reaction regime, where theheat evolved by the exotherm of the decomposition reaction is utilizedfor heating the catalyst bed, the gas inlet temperature on entry intostage B is 200-700° C., preferably 300-600° C., preferably 400-550° C.,particularly preferably 430-550° C., in particular 450-500° C. The gasinlet temperature can depend on the activity of the catalyst.

To minimize the thermal formation of NO_(x) and to protect the catalystused from destruction due to excessive temperatures (e.g. by sintering),the temperature of the gas stream on exit from the reactor (stage B)should not significantly exceed 800° C. This can be achieved for exampleby the concentration of N₂ O in the gas stream entering step B not beingmore than 40% by volume, preferably within the range from 0.1 to 20% byvolume, particularly preferably within the range from 0.5 to 15% byvolume, in particular within the range from 1 to 13% by volume. Gasstreams often contain N₂ O-contents of more than 20% by volume.

This reduction of the N₂ O concentration can be achieved for example byadmixing the gas stream with an essentially N₂ O-free gas streamupstream of stage B. The admixing can also be carried out upstream ofstage A, if the gas stream first passes through stage A. The essentiallyN₂ O-free gas stream can be the gas stream leaving stage B or, asexplained below, optionally the gas stream leaving stage C and/or a gasstream containing free oxygen and/or a process gas.

The N₂ O removal can also be carried out isothermally. This is possiblefor example in a tube bundle reactor with salt bath or metal bathcooling. This process is characterized in that the temperature of thegas stream on exit from the reactor (stage B) corresponds to thetemperature of the salt or metal bath and the molten salt or metalabsorbs the heat released by the N₂ O decomposition reaction. The saltor metal bath temperature is preferably 400-650° C. or corresponds tothe temperature of the adiabatic reaction regime. The gas stream can beheated up either upstream of stage B by a heat exchanger, such as agas/gas heat exchanger, or directly in the salt or metal bath reactor ofstage B.

Another possibility is the removal of N₂ O (decomposition) in afluidized bed.

Catalysts

Examples of catalysts suitable for N₂ O removal by catalyticdecomposition are the catalysts described in DE 43 01 470, DE 42 24 881,DE 41 28 629, WO93/15824, EP 625369, WO94127709, U.S. Pat. No.5,171,553. Suitable catalysts may consist for example of CuO, ZnO andAl₂ O₃ or additionally include Ag. It is possible to use catalysts withAg as active component applied to a gamma-Al₂ O₃ support. Furtherexamples of usable catalysts are those having CoO and/or NiO on a ZrO₂support. The use of zeolitic catalysts, for example mordenites, whichare present in the H⁺ or NH₄ ⁺ form and may be exchanged with V, Cr, Fe,Co, Ni, Cu and/or Bi is likewise possible.

Also suitable are catalysts consisting of zeolites having an SiO₂ /Al₂O₃ ratio of at least 550, for example beta zeolite, ZSM-5, 4 zeolite,mordenite or chabazite and are present in the H⁺ or NH₄ ⁺ form andoptionally exchanged with alkali, alkaline earth, transition metals orrare earth elements, in which case cobalt can be preferred asparticularly suitable.

Likewise usable are catalysts based on zeolite which have been exchangedwith Cu, Co, Rh, Pd or Ir, for example.

Other catalysts which make possible the reduction or decomposition of N₂O are likewise usable.

As well as catalytic reduction or decomposition of N₂ O, thermaldecomposition is also possible, for example in a regenerative heatexchanger (thermoreactor).

Stage C

In a preferred embodiment of the present invention, the gas stream fromstages A and B can be passed through a stage C for reducing nitrogenoxides other than N₂ O.

The decomposition of N₂ O in stage B may in certain circumstances leadto the formation of nitrogen oxides NO_(x). These newly formed nitrogenoxides can preferably be removed in stage C.

Stage C is for the reduction of nitrogen oxides other than N₂ O.

In stage C the gas stream can be reacted by means of selective catalyticreduction (SCR), for example. In SCR, the nitrogen oxides are reactedwith ammonia as reducing agent over catalysts. DENOX catalysts can beused for example. The products are nitrogen and water.

Stage C may also be run as a nonselective catalytic reduction (NSCR).NSCR involves the use of hydrocarbons to reduce the nitrogen oxides andcatalysts containing noble metals.

SCR and NSCR processes are described for example in Ullmann'sEncyclopedia of Chemical Technology, loc. cit.

The catalysts used in this process can be any desired suitablecatalysts. For example, catalysts for nonselective reduction processescan be based on platinum, vanadium pentoxide, iron oxide or titanium.Selective catalytic reduction catalysts may contain for example noblemetals, such as Pt, Rh, Ru, Pd and/or metals of the iron group, such asFe, Co, Ni. It is also possible to use, for example, vanadium pentoxide,tungsten oxide or molybdenum oxide. A further suitable catalyst isvanadium pentoxide on an alumina support.

The nonselective reduction process may involve the use of suitablehydrocarbons, such as natural gas, propane, butane, naphtha, but alsohydrogen.

The temperature of the gas stream on entry into stage C can be forexample 150-500° C., preferably 200-350° C., particularly preferably260-300° C.

It was found according to the present invention that the reactions ofstages A, B and, if employed, C can preferably be carried out on onepressure level. This means that the pressure of the gas stream is notadditionally significantly increased or reduced between the individualstages. The pressure is at least 3 bar, preferably within the range from3 to 20 bar, preferably 3 to 12 bar, particularly preferably within therange from 5 to 10 bar.

Stages A, B and, if employed, C can thus be accommodated in anintegrated pressure apparatus consisting of the two or three, as thecase may be, reactors, ie. as an integrated unit in which the gas streamis brought to the starting pressure prior to entry into one of thestages, for example by compression, and between the individual stagesthere are no further means whereby the pressure of the gas stream issignificantly increased or reduced. As the gas stream passes through thestages, the to pressure in the gas can vary as a function of the stagesused. Preferably, however, the pressure of the gas stream is not variedbeyond that. On exit from the last stage the gas stream can be broughtto atmospheric pressure, for example by means of a decompressionturbine.

Conducting the entire process at one pressure level allows simpleprocess control and a simplified construction of the entire apparatusfor removing nitrogen oxides. Process control can be greatly simplifiedas a result.

In a preferred embodiment, the gas stream passes through stages A, B, C,preferably in that order, and before entry into stage A is admixed withair and/or a gas stream leaving B or C and/or a process gas so that theN₂ O content is preferably not more than 20% by volume.

The gas stream is contacted in stage A with water or aqueous solutionse.g. of nitric acid, in an absorption column in countercurrent to formHNO₃ and the product HNO₃ is removed at the base of the column,

then the remaining gas stream is brought to a temperature of 200-700°C., preferably 450-500° C. and contacted in stage B in a fixed bed witha catalyst for catalytic decomposition of N₂ O,

the remaining gas stream is then brought to a temperature of 150-500°C., preferably 260-300° C. and subjected in stage C to a catalyticreduction.

The heat of reaction evolved in the individual stages can be utilizedfor generating steam and mechanical drive energy. For example, the gasstream can be brought upstream of stage A to a pressure of from 1,5 to20 bar absolute by means of a compressor (V1) and downstream of stage Cto ambient pressure by means of an expansion turbine (T1), in which casethe energy released in the expansion turbine (T1), as can be providedfor example by a motor or engine, is supplied to the compressor (V1)with or without farther energy (M).

The energy released in the individual reaction stages can also be usedfor preheating the gas stream.

For example, the gas stream, before entry into stage A, can be cooled ina heat exchanger (WT1) with the gas stream emerging from stage A.Similarly, the gas stream, before entry into stage B, can be heated in aheat exchanger (WT3) with the gas stream emerging from stage B. Inaddition, the gas stream, downstream of the heat exchanger (WT1) andbefore entry into stage A, can be additionally further cooled to thedesired temperature with a further heat exchanger (WT2). Furthermore,the gas stream, downstream of the heat exchanger (WT3) and before entryinto stage C, can be additionally further cooled with a heat exchanger(WT4).

As well as the process for removing nitrogen oxides from a gas streamcontaining same, the present invention also provides an apparatustherefor. The apparatus comprises the above-described stages A, B andpreferably the above-described stages A, B and C, preferably in thatorder. According to one embodiment of the invention other orders of thesteps, e.g. BAC, ACB and the like are possible.

The individual stages in the apparatus are preferably interconnectedusing suitable lines in such a way that the gas stream can pass throughthe stages in succession.

Preferably, the apparatus for removing nitrogen oxides from a gas streamcontaining same includes, upstream of the first stage, an apparatuswhereby the gas stream can be brought to a desired pressure and nofurther apparatus for additionally significantly increasing or reducingthe pressure of the gas stream between the individual stages.

In a preferred embodiment, the apparatus comprises the above-describedcompressor (V1) and expansion turbine (T1) and also a motor/engine (M),as described above.

In a further preferred embodiment of the apparatus, it comprises theheat exchangers (WT1) and (WT3) arranged as described above.

In a further preferred embodiment of the apparatus, it comprises theheat exchangers (WT2) and (WT4) arranged as described above.

The present invention also relates to the use of the above-describedapparatus for removing nitrogen oxides from a gas stream containingsame. The gas stream in question preferably comprises a waste gas streamfrom processes for producing adipic acid, nitric acid, hydroxylaminederivatives or caprolactam or from processes for burning nitrogenousmaterials.

The present invention further provides for the use of theabove-described apparatus for producing HNO₃.

A preferred apparatus according to the present invention and a preferredprocess according to the present invention will now be described withreference to the drawing which is a diagram of an apparatus according tothe present invention.

The reference symbols in the drawing have the following meanings:

    ______________________________________                                        K1:        absorption column (stage A)                                        C1:        N.sub.2 O-cracking reactor (stage B)                               C2:        reactor for catalytic NO.sub.x reduction (stage C)                 WT1:       heat exchanger 1                                                   WT2:       heat exchanger 2                                                   WT3:       heat exchanger 3                                                   WT4:       heat exchanger 4                                                   V1:        compressor                                                         T1:        expansion turbine                                                  M:         motor/engine                                                       ______________________________________                                    

The numerals signify the individual gas streams.

EXAMPLE

In an apparatus constructed according to the accompanying drawing,process and waste gases containing nitrogen oxides (line 1) are mixedvia line 2 with air and/or via line 3 with N₂ O-lean or NO- and NO₂-containing process gases. The admixture of air and N₂ O-lean or -freeprocess gas limits the temperature increase due to the adiabaticallyoperated N₂ O decomposition in the downstream reactor C1 to maximum 350°C. In addition, air is admitted to support the oxidation of NO accordingto the above-recited equation (I) and thus the formation of nitric acidaccording to equation (II) in the absorption column K1. The productionof nitric acid (HNO₃) in the absorption column K1 can be additionallyincreased by the addition of NO- and/or NO₂ -containing gases. In apreferred embodiment, process gases from ammonia oxidation reactors canbe fed in via line 3.

The gas mixture (the gas stream containing nitrogen oxides) is thencompressed by means of the compressor (V). The resulting increasedpressure of the gas mixture considerably improves the effectiveness ofthe downstream absorption column K1 (stage A) of the N₂ O-crackingreactor C1 (stage B) and of the reactor for catalytic NO_(x) reductionC2 (stage C) in a preferred embodiment. The evolved heat of compressionand the simultaneous oxidation of NO to NO₂ increases the temperature ofthe gas stream in line 4 to 250-350° C. The gas stream is cooled down to30-40° C. in a gas/gas heat exchanger (WT1) with cold gas stream fromthe absorption subsequently in the heat exchanger (cooler) (WT2) with asuitable cooling medium such as air or cooling water.

The NO₂ absorption and reaction with water to form nitric acid iscarried out in the downstream absorption column K1 (stage A), where thegas stream and the absorbent (e.g. water or aqueous nitric acid) arepassed countercurrently over suitable internal fitments and theresulting nitric acid is withdrawn from the base of the column.

The gas stream (line 6) freed from the bulk of the NO₂ and NO is thenheated in a gas/gas heat exchanger (WT1) to 200-300° C. (line 7) and inthe downstream gas/gas heat exchanger (WT3) to 450-500° C. (line 8). Theremoval of the N₂ O takes place in reactor C1 (stage B), the temperaturerising to up to 825° C. (line 9). The gas stream is then cooled down ingas/gas heat exchanger (WT3) and subsequently in the steam generator(heat exchanger WT4) to 260-300° C. (line 10). Then the gas stream isfreed by catalytic reduction from remaining nitrogen oxide traces inreactor C2 (stage C) by catalytic reduction. In the case of NO_(x)contents of the waste gas of 1000 ppm the adiabatic temperature increaseis about 10° C. The gas stream is then fed via line 11 at a temperatureof 265-310° C. to an expansion turbine (Ti), where it is decompressed toatmospheric pressure and released into the atmosphere at about 100° C.via line 12.

The drive energy generated in turbine (T1) can be utilized, via a commonshaft, for driving the compressor (V1). The missing drive energy is thenadditionally supplied via an additional motor/engine (M).

What is claimed is:
 1. A process for removing nitrogen oxides from a gasstream in which they are contained which comprises passing the gasstream(A) through a stage for absorbing the nitrogen oxides other thanN₂ O in an absorbent or reacting the nitrogen oxides other than N₂ Owith an absorbent, and (B) through a stage for reducing the amount of N₂O, and, after stages A and B, (C) through a stage C for reducingnitrogen oxides other than N₂ O, wherein the gas stream passes firstthrough stage A and then through stage B, and wherein steps A, B and Care carried out at essentially the same pressure level within the rangeof from 3 to 20 bar.
 2. A process as claimed in claim 1, wherein, instage A, the absorbent used is water or an aqueous solution of nitricacid and the nitrogen oxides other than N₂ O are converted into HNO₃ inthe presence or absence of free oxygen.
 3. A process as claimed in claim1, wherein, in stage B, the reduction of the amount of N₂ O is effectedby thermal and/or catalytic decomposition.
 4. A process as claimed inclaim 1, wherein the gas stream is brought upstream of stage A to apressure of from 3 to 20 bar absolute by means of a compressor (V1) anddownstream of stage C to ambient pressure by means of an expansionturbine (T1) and the energy released in the expansion turbine (T1) issupplied to the compressor (V1) with or without further energy (M),wherein, optionally, the gas stream before entry into stage A is cooledin a heat exchanger (WT1) with the gas stream emerging from stage A and,before entry into stage B, heated in a heat exchanger (WT3) with the gasstream emerging from stage B, and wherein, optionally, the gas stream,downstream of the heat exchanger (WT1) and before entry in to stage A,is additionally further cooled with a heat exchanger (WT2) and the gasstream, downstream of the heat exchanger (WT3) and before entry intostage C, is additionally further cooled with a heat exchanger (WT4).